1. Field of the Invention
The present invention relates to a method for speed stage shifting of an automatic transmission in a vehicle such as an automobile, and, more particularly, to a control of the back pressure of an accumulator connected to a passage supplying a hydraulic fluid to a hydraulically operated friction engaging means for shifting the automatic transmission from one speed stage to another speed stage.
2. Description of the Prior Art
In the speed stage shifting of an automatic transmission in a vehicle such as an automobile in which a second hydraulically operated friction engaging means is put into engagement in exchange with disengagement of a first hydraulically operated friction engaging means, in order to accomplish the speed stage shifting with minimum rate of change of the output torque of the transmission, it is essential that the torque load applied to the second friction engaging means is gradually increased so that the second friction engaging means is brought into full engagement through a certain transition period of sliding half engagement. In order to provide such a transitional sliding engagement an accumulator is generally connected to a passage supplying a hydraulic fluid to the friction engaging means. Such an accumulator maintains the hydraulic pressure supplied to the friction engaging means at an intermediate pressure level for a certain period as its piston member incorporated therein makes a stroke by absorbing a part of the hydraulic fluid supplied toward the friction engaging means. The piston of the accumulator is generally supported by a spring and a back pressure supplied to a rear side of the piston opposite to its front side exposed to the hydraulic pressure supplied toward the friction engaging means. It is known to control the transient intermediate pressure level of the hydraulic pressure supplied to the friction engaging means by the control of the back pressure of the accumulator in accordance with changes of the torque load imposed on the friction engaging means according to changes of the operating conditions of the transmission.
However, such a control of the back pressure of the accumulator in the conventional art is to change its pressure level according to the loading conditions of the transmission in such a long term base that includes the whole period of the speed stage shifting as a part thereof. In other words, the conventional control of the back pressure of the accumulator is such that the back pressure is modified according to the throttle opening of the engine so that the torque load imposed on the transmission is reflected upon the transient intermediate pressure level provided by the accumulator during the progress of the speed stage shifting.
However, it is noted that for a more improved smooth speed stage shifting of the transmission it is essential that the progress of engagement at the very moment of transfer of the friction engaging means from its disengaged state into its slidingly half engages state is still further reduced to more gradual manner.